Generally, a vehicular automatic transmission comprises two linked planetary gear units, and planetary gearing for reducing speed of the rotation received from the input shaft (for example, see Japanese Unexamined Patent Application Publication No. 4-125345). Such a transmission achieves, for example, six forward speeds and one reverse speed, by input of reduced speed rotation from the planetary gearing via a clutch to, for example, one rotary component of a planetary gear unit that has four rotary components. Further, in the case of fifth speed forward, for example, when the rotation of the input shaft is simultaneously input into two of the rotary components of the planetary gear unit by engaging two clutches, this fifth speed forward is a directly coupled state, with rotation similar to that of the input shaft (see Japanese Unexamined Patent Application Publication No. 2000-274498, for example).
The above-described automatic transmission comprises two clutches for input of the rotation of the input shaft into two of the rotary components of the aforementioned planetary gear unit, and planetary gearing for reducing the speed rotation input into the rotary components of the planetary gear unit. However, if those two clutches or the oil servos that control the engagement of those clutches are located between the planetary gear unit and the speed-reduction planetary gearing, the element(s) for transmitting the reduced speed rotation of this speed-reduction planetary gearing to the rotary components of the planetary gear must be axially lengthened.
The axial lengthening of the element(s) that transmits the reduced speed rotation means that the unit transmitting a large torque is elongated, and an elongated unit that can withstand the large torque requires elongation of a relatively thick material, preventing a compact automatic transmission. Further, the weight of such a unit would be heavier, and not only would a lightweight automatic transmission become impossible, but inertia (inertial force) would increase, thereby reducing the controllability of the automatic transmission and speed change shocks would become more likely to occur.
In order to engage or disengage the reduced speed rotation output to the planetary gear unit from the speed reduction planetary gearing, a clutch or brake must be provided. When a clutch is provided, this clutch and the above-mentioned two clutches, in other words three clutches, are necessary. In general, a clutch has a drum-shaped member (clutch drum) that transmits the input rotation to the friction plates, and therefore, for example with a problem such as relative rotation, supply of oil pressure to the oil compartment of the hydraulic servo of the clutch must come from the mid-section of the automatic transmission.
However, if the required three clutches are located on one axial side of the planetary gear unit, oil lines for supplying oil pressure to three hydraulic servos are provided in triplicate in the mid-section of the automatic transmission, and the configuration of the oil lines becomes complicated.